You're pretty much south of London city right?
If so it could be the establishing turn that you see. In other words the turn onto the ILS (instrument landing system) to establish the approach for one of the westerly runways.
That would be my guess looking at Bromley on a map and trying to estimate where the Heathrow RMA is. I'm sure a more proficient answer will come with time though, but perhaps my guess is a start anyway
